Late call forwarding method in IP multimedia core network subsystem centralized service

ABSTRACT

A late call forwarding method in the IMS centralized service. During a call setting up, an ICCF receives a call request from the CSCF serving the called user, called session A, and the ICCF acquires a called user&#39;s roaming number and initiates a call to the called user. After receiving the call request, the VMSC serving the called user triggers the late call forwarding service according its determination. A CAMEL service is triggered when the VMSC initiates a call to the forwarding target. The VMSC exchanges messages with the ICCF and informs the ICCE about session associated information and the forwarding reason. When the ICCF determines that the forwarding is needed, it sends a response to the session A does not send the response message, after a telecom application server (TAS) receives the indication, the TAS initiates a regular forwarding service.

TECHNICAL FIELD

The present invention relates to the technology of accessing to the IPMultimedia Subsystem (IMS) via the circuit switching domain, and moreespecially, to a late call forwarding method in the IMS centralizedservice.

BACKGROUND OF THE RELATED ART

Internet Protocol Multimedia Subsystem (IMS), which is an IP-basednetwork architecture proposed in the 3^(rd) Generation PartnershipProject (3GPP), constructs an open and flexible service environment,supports multimedia applications and provides abundant multimediaservices for users.

IMS, which is an IP-based telecom network architecture and isindependent of access technology, can provide services for mobilecellular networks such as Global System for Mobile communications (GSM)and Universal Mobile Telecommunications System (UMTS), and so on, inaddition to providing services for the Packet Switching access networkssuch as the General Packet Radio Service (GPRS), Wireless Local AreaNetwork (WLAN), and so on.

The mobile cellular network, such as GSM and UMTS, applies the circuitswitching technology, which is called a Circuit Switching (CS) domainand able to provide basic voice services and supplementary servicesbased on voice services for users. When accessing to the IMS, the IMSevolves into an access mode, where services are provided entirely by theIMS. Such technology is called IMS Centralized Service (ICS for short).

The IMS centralized services have the following advantages:

(1) the IMS provides uniform services for access modes, such as circuitswitching domain and packet switching domain, and supports networkconvergence;

(2) it supports the evolvement of a CS network into an IMS network; and

(3) it supports both a UE with ICS capability and an existing UE withoutICS capability.

FIG. 1 is an illustration of an application scenario of session paths ofthe IMS centralized service, and it comprises the following networkelements:

101: a User Equipment (UE);

102: a Visited Mobile Switch Center (VMSC);

103: a Home Subscriber Server (HSS);

104: a Media Gateway Control Function (MGCF);

105: a Media Gateway (MGW);

106: an IMS CS Control Function (ICCF);

107: a Call Session Control Function (CSCF);

108: a Telecom Application Server (TAS); and

109: a GSM Service Control Function (gsmSCF).

There are three paths between the UE 101 and the ICCF 106 to beestablished: a session control path, a bearer control path and a bearerpath.

Wherein, the session control path comprises the following two modes:

(1) The CS session control path: which is in a CS domain and usesUnstructured Supplementary Service Data (USSD), and this path is via theVMSC 102 and the HSS 103;

(2) The PS session control path: which is in a PS domain and uses aSession Initiation Protocol (SIP) signaling, and the path is via theCSCF 107.

In the bearer control path, the UE 101 accesses to the VMSC 102 usingstandard CS control signaling and accesses to the IMS through the MGCF104, and reaches the ICCF 106 through the CSCF 107.

In the bearer path, the UE 101 accesses to the IMS through the VMSC 102and the MGW 105 and establishes a media connection with a remote UE ofthe session.

The IMS centralized services utilize the session control path betweenthe UE 101 and the ICCF 106 to exchanges the session control message,and establish and control media bearer through the bearer control path,where the ICCF 106 acts as a User Agent (UA) of the IMS to access to theIMS on behalf of the UE.

The TAS 108 is the telecom application server in the IMS, and the basictelecom services are implemented in the TAS 108.

The function of the gsmSCF 109 is: when the VMSC triggers the mobileintelligent services (CAMEL, Customized Applications for Mobile networkEnhanced Logic) to the gsmSCF 109, the gsmSCF 109 distributes theservices request to the ICCF 106 according to the service information.

To date, there are mainly the following types of call forwarding in CS:

1) Call forwarding on unreachable

It is divided into the forwarding on phone powered off and theforwarding on no paging response and all incoming calls which call theUE are forwarded to a preset number.

2) Call forwarding on busy

The Call forwarding on busy is divided into forwarding onnetwork-determined user busy and forwarding on user-determined userbusy. Wherein, in the forwarding on network-determined user busy, when aUE is in a call, the other incoming calls are automatically forwarded tothe preset number; in the forwarding on user-determined user busy, whena called user rings, the called user sends the user-busy message to theVMSC 102 depending on personal willingness thereof, and the VMSC 102triggers the forwarding according to the message.

3) Call forwarding on no answer

When the phone is powered on and the incoming call is connected but isnot answered, then the incoming call is automatically forwarded toanother number preset.

4) Call forwarding unconditional

All phone numbers that call the UE are forwarded to another numberpreset.

In the CS domain, the forwarding service can be classified into theearly forwarding and late forwarding, wherein, the early forwarding isthat when acquiring the called user's roaming number, the home locationregister sends the forwarding message to the gateway mobile switchcenter to let the gateway mobile switch center trigger the forwardingservice, such as the call forwarding on no conditions, call forwardingon phone powered off; while the late forwarding is that the callconnects to the VMSC 102, but the call forwarding service is stilltriggered due to the call release by the user, such as the above callforwarding on busy, the call forwarding on no paging response, and thecall forwarding on no reply etc.

In the present IMS centralized service, there is no complete solutionfor the forwarding on user-determined user busy, the forwarding on nopaging response and the call forwarding on no reply etc.

Content of the Invention

The present invention provides a late call forwarding method in the IMScentralized service to overcome the shortcoming of the prior art andimplement the late call forwarding on the basis of the network structureof the present IMS centralized service.

In order to solve the above problem, the present invention provides alate call forwarding method in an IP multimedia subsystem centralizedservice (ICS), and the method comprises the following steps of:

A1, during a call, an internet protocol multimedia subsystem circuitswitch control function (ICCF) receiving a call request from a callsession control function (CSCF) serving a called user, and the callbeing referred to as session A; the ICCF acquiring a called user'sroaming number and initiates a call, being referred to as session B, tothe called user;

A2, after receiving a call request of the session B, a visited mobileswitch center (VMSC) serving the called user determining that a statusof the called user satisfies conditions for triggering late callforwarding and the called user subscribes a late call forwarding servicehas the Originating-CAMEL Subscription Information (O-CSI) for thecalled user, then the VMSC triggering the late call forwarding serviceand triggers an intelligent service when initiating a call to aforwarding target; and

A3: said VMSC exchanging messages with said ICCF to inform said ICCFabout session associated information and a forwarding reason, and whensaid ICCF determines that the forwarding is needed, said ICCF sending aresponse including corresponding indication to the session A or does notsending a response depending on the forwarding reason, and said CSCFrelaying the indication to a telecom application server (TAS), and theTAS initiating a regular forwarding service.

Furthermore, in the step A2, said conditions for triggering said latecall forwarding comprises call forwarding on user-determined user busy,call forwarding on no paging response or call forwarding on no answer.

Furthermore, in the step A3, said VMSC exchanging the intelligentservice message with said ICCF comprises the following steps of:

A311. said VMSC sending the intelligent service message including thesession associated information and the forwarding reason to the globalsystem for mobile communications service control function (gsmSCF), andsaid gsmSCF forwarding the message to said ICCF; and

A312. after said ICCF determining that the message relates with thesessions A and B according to the session associated information in thereceived intelligent service message, said ICCF determining that theforwarding is needed and then sending a customized applications formobile network enhanced logic (CAMEL) failure message to said VMSC, andthe failure message reaching said VMSC via said gsmSCF.

Furthermore, in the step A3, said VMSC exchanging the intelligentservice message with said ICCF comprises the following steps of:

A321. said VMSC sending the intelligent service message including thesession associated information and the forwarding reason to the gsmSCF,and said gsmSCF forwarding the message to said ICCF;

A322. after said ICCF determining that the message associates with thesessions A and B according to the session associated information in thereceived intelligent service message, said ICCF allocating a temporaryIMS routing number (IMRN) and associating the IMRN with said sessions Aand B, storing the forwarding reason, and then returning the IMRN tosaid VMSC through CAMEL connection message, and said CAMEL connectionmessage reaching said VMSC via said gsmSCF;

A323. after receiving the CAMEL connection message, said VMSC takingsaid IMRN as a called number to initiate a session C;

A324. after receiving a call request for the session C, said ICCFdetermining that the session C associates with the sessions A and Baccording to said IMRN and that the forwarding is needed, then sendingthe response message to CSCF in the session A including correspondingindication.

Furthermore, in the step A311 or A321, said intelligent service messageincluding the session associated information and the forwarding reasonis an Initial Detection Point (IDP) message.

Furthermore, said forwarding reason is indicated in a field of thecalled number or a forwarding reason in said IDP message.

Furthermore, said session associated information is a calling number oran original called number in the IDP message; alternatively, saidsession associated information is the calling number and the originalcalled number in the IDP message.

Furthermore, step A3 applies the following way to release the session B:

After determining that the received intelligent service messageassociates with the sessions A and B, said ICCF releasing the session B;

Alternatively, after receiving a customized applications for mobilenetwork enhanced logic (CAMEL) failure message from said ICCF, said VMSCreleasing the session B;

Alternatively, after said ICCF determines that the received intelligentservice message associates with the sessions A and B, said ICCFreleasing the session B; meanwhile, after said VMSC receives the CAMELfailure message from said ICCF, said VMSC also releasing the session B.

Furthermore, the step A3 applies the following way to release thesessions B and C:

After determining that the session C associates with the sessions A andB, said ICCF releasing the sessions B and C;

Alternatively, after said ICCF determines that the session C associateswith the sessions A and B, said ICCF releasing one of the sessions B andC, and after said VMSC receives the release message, said VMSC releasingthe other session;

Alternatively, after said ICCF determines that the session C associateswith the sessions A and B, said ICCF releasing the sessions C and B; andafter said VMSC receives a first arriving releasing message of either ofthe sessions C and B, said VMSC releasing the other session.Furthermore, in the step A3, when the forwarding reason is callforwarding on no answer, said ICCF returns a corresponding sessioninitiation protocol (SIP) failure response message in the session A, andafter said TAS receives the message, said TAS initiates a callforwarding on no answer service according to said SIP failure responsemessage; alternatively, said ICCF does not send any response message tothe TAS, and the TAS waits for an expiration and triggers the callforwarding on no answer;

Alternatively, if the forwarding reason is a call forwarding onuser-determined user busy or call forwarding on no paging response, saidICCF returns a corresponding SIP failure response message to the sessionA, and after said TAS receives the message, said TAS initiates the callforwarding on user-determined user busy or the call forwarding on nopaging response according to the SIP failure response message.

Furthermore, in the step A2, when the called user registers in a circuitswitching domain, subscription data of the called user which includeslate call forwarding service and the O-CSI are downloaded to said VMSC,and said VMSC triggers said late call forwarding service and theintelligent service according to the subscription data and the status ofthe called user.

The technical effects of the present invention are as follows:

1. The present invention provides a method for implementing the latecall forwarding in the IMS centralized service to implement the latecall forwarding with a relatively simple message interaction processbased on the present network architecture and the communicationstandard.

2. The present invention sends the forwarding reason to the IMS domainthrough the method of the calling intelligent service to control thelate call forwarding service in the IMS domain, thus achieve the IMScentralized service control of the late call forwarding.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of the session path in an application scenarioof the IMS centralized service in the prior art;

FIG. 2 is a flow chart of implementing the late call forwarding throughthe ICS service in accordance with the first embodiment of the presentinvention;

FIG. 3 is a flow chart of implementing the late call forwarding throughthe ICS service in accordance with the second embodiment of the presentinvention.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

The present invention provides a solution for the late call forwardingin the ICS, and the solution does not consider the scenario onnetwork-determined user busy since this scenario can be directlydetermined and triggered by the TAS.

In the present invention, the called user (UE-B) subscribes the latecall forwarding service in the CS domain and O-CSI. When the userregisters in the CS domain, the subscription data of the late callforwarding service and the O-CSI are downloaded to the VMSC.

The technical scheme of the present invention will be described indetail in the following in conjunction with the accompanying figures andthe preferred embodiments, and the embodiments of FIG. 2 and FIG. 3comply with this process.

FIG. 2 is a flow chart of implementing the late call forwarding in theICS service. When the CSCF serving the called user receives a callrequest, it sends the call request to the ICCF via the TAS according tothe initial filter criterion. The ICCF acquires the called user'sroaming number and then initiates a call to the called user, and thecall reaches the VMSC. The VMSC triggers the late call forwardingservice if the current status of the called user satisfies conditions ofthe late call forwarding, initiates a call to the forwarding target,triggers the calling CAMEL service and sends the CAMEL message to theICCF; after the ICCF determines that the forwarding is needed, ittranslates the message into the SIP indication message and sends it tothe TAS, and the TAS triggers the late call forwarding service accordingto the SIP indication message.

The detailed process is shown as follows:

S201, the CSCF serving the called user receives a SIP call request fromthe calling side, and after the call request is processed by TASaccording to an initial filter criterion, the CSCF sends it to the ICCF,the session received by the ICCF is referred to as session A.

S202, the ICCF here works as the B2BUA (Back to Back User Agent), and itacquires the UE-B's roaming number first.

S203, the ICCF continues to establish the session according to theUE-B's roaming number by sending a SIP call request message, whichcarries the roaming number, to the MGCF via the CSCF, the session beingreferred to as session B.

S204, the MGCF sends an integrated services digital network user part(ISUP) initial address message to the VMSC serving the UE-B.

S205, after the VMSC receives the ISUP initial address message, itdetermines that the current status of the UE-B satisfies conditions oflate call forwarding and triggers the late call forwarding, and then theVMSC initiates a call to the forwarding number according to the usersubscription data.

S206, when the VMSC initiates a call, the calling CAMEL service of theUE-B is triggered.

This call only performs the first part of the process of setting up thecall to trigger the calling CAMEL service. Since the UE-B subscribes thecalling CAMEL service, so the VMSC has the calling CAMEL service data ofthe calling user.

S207, the VMSC sends a CAMEL IDP message including the sessionassociated information (might be the original called number (UE-Bnumber), the calling number, or both of them) and the forwarding reasonto the gsmSCF, and the gsmSCF forwards the IDP message to the ICCF.

Wherein, the forwarding reason may be forwarding reason in the CAMEL IDPmessage or can be determined according to the particular called number.

S208, the ICCF determines that it associates with the session A receivedin S201 according to the session associated information in the CAMEL IDPmessage and returns a corresponding SIP failure response message to thesession A according to the forwarding reason, and the message reachesthe TAS via the CSCF.

Here, if the ICCF finds out that it is the call forwarding on no answer,it may not send any response message to the TAS, and the TAS triggersthe call forwarding on no answer when the time for the session expires.

S209, the TAS triggers a regular late call forwarding service accordingto the SIP failure response message.

S210, the ICCF sends a CAMEL failure message to the VMSC, and the CAMELfailure message reaches the VMSC via the gsmSCF.

S211, the VMSC releases the session B received in S204 and sends an ISUPrelease message to the MGCF.

S212, the MGCF sends a SIP session cancellation message to the ICCF viathe CSCF, and the ICCF releases the session B after receiving themessage.

It should be noted that there is no order for S210 and S208, and theycan be processed in parallel.

There are many ways to release the session between the ICCF and the VMSCafter S207, and the above embodiment just applies one of them; anotherone is that the ICCF releasing the session B after S207; still anotherone is that the ICCF and the VMSC release the session respectively, inwhich the ICCF deletes the session established in S203 and S204 afterS207 and the VMSC deletes the session established in S203 and S204 afterreceiving the message in S210, and this method is more reliable.

It should be noted that the above forwarding reason may be the callednumber field in the CAMEL IDP message. For example, different particularlate call forwarding number in the CS domain is used as the identifierof the forwarding reason, and the particular late call forwarding numberis filled into the called number field in the CAMEL IDP messageaccording to the forwarding reason during the late call forwarding.Moreover, the forwarding reason may be the forward reason field in theCAMEL initial Detection point message.

FIG. 3 is a flow chart of implementing the late call forwarding throughthe ICS service in accordance with the second embodiment. When a callrequest arrives at the CSCF serving the called user, the CSCF sends thecall request to the ICCF via the TAS according to the initial filtercriterion. The ICCF acquires the called user's roaming number and theninitiates a call to the called user, and the call reaches the VMSC. Ifthe current status of the called user satisfies the condition fortriggering the late call forwarding, the VMSC initiates a call to theforwarding target, triggers the calling CAMEL service and sends theCAMEL message to the ICCF; after the ICCF determines that the forwardingis needed, it store the forwarding reason and allocates a temporary IMSrouting number (IMRN). The ICCF returns the IMRN to the VMSC through theCAMEL connection message. The VMSC uses the IMRN to initiate a call, andafter the call reaches the ICCF, the ICCF acquires the forwarding reasonaccording to the association, and translates it to the indication incorresponding SIP message and sends it to the TAS, and the TAS triggersthe late call forwarding service according to the SIP indicationmessage.

The detailed process is shown as follows:

S301, the CSCF serving the called user receives the SIP call requestfrom the calling side, and after the SIP call request is processed bythe TAS according to the initial filter criterion, the CSCF sends it tothe ICCF, the session received by the ICCF is referred to as session A.

S302, the ICCF here works as the B2BUA (Back to Back User Agent), and itacquires the UE-B's roaming number first.

S303, the ICCF continues to establish the session according to theUE-B's roaming number and sends a SIP call request message to the MGCF,and the call is referred to as the session B, the SIP carrying theroaming number reaches the MGCF via the CSCF.

S304, the MGCF sends an ISUP initial address message to the VMSC servingthe UE-B.

S305, if the current state of the UE-B satisfies the condition fortriggering the late call forwarding in the VMSC, the late callforwarding of the UE-B is triggered, and the VMSC initiates a call tothe forwarding number according to the user subscription data.

S306, when the VMSC initiates a call, since the VMSC has calling CAMELservice data, the calling CAMEL service of the UE-B is triggered.

S307, the VMSC sends a CAMEL initial monitoring point message includingthe session association information (might be the original called number(UE-B number), the calling number, or both of them) and the forwardingreason to the gsmSCF, and the gsmSCF forwards the message to the ICCF.

Wherein, the forwarding reason may be the forward reason in the CAMELinitial monitoring point message or may be determined according to thespecific called number.

S308, the ICCF determines that it associates with the session in S303and S301 according to session associated information in the CAMEL IDPmessage. The ICCF allocates a temporary IMS routing number (IMRN) toassociate it with the sessions in S303 and S301, and stores theforwarding reason. The ICCF returns the IMRN to the VMSC through theCAMEL connection message, and the message reaches the VMSC via thegsmSCF.

S309, after the VMSC receives the CAMEL connection message, it sends anISUP initial address message according to the IMRN in the message, andthe called number is the received IMRN, and the message reaches the MGCFfirst.

S310, the MGCF sends an SIP call request message to the ICCF, and thecall is referred to as the session C, and the called number is thetemporary IMS routing number, and the SIP call request message reachesthe ICCF via the CSCF.

S311, the ICCF determines that it associates with the session in S301and S303 according to the IMRN in the received SIP call request message.The ICCF returns a corresponding SIP failure response message to thesession A received in S301 according to the stored forwarding reason,and the message reaches the TAS via the CSCF.

Similarly, if the ICCF finds out that here it is the call forwarding onno answer, it will not send any response message to the TAS, and the TAStriggers the call forwarding on no answer when the timer for the sessionexpires.

S312, the TAS triggers the late call forwarding service according to theSIP failure response message.

S313, the ICCF cancels the session B established in S303, and sends aSIP cancellation message to the MGCF via the CSCF.

S314, the MGCF sends an ISUP release message to the VMSC, and the VMSCreleases the session B after receiving the message.

S315, the ICCF cancels the session C established in S310, and sends anSIP cancellation message to the MGCF via the CSCF.

S316, the MGCF sends an ISUP release message to the VMSC, and the VMSCreleases the session C after receiving the message.

It should be noted that there is no order for S311, S313 and S315, andthey can be processed in parallel.

There are many ways to release the two sessions between the ICCF and theUE-B after S310, wherein, one of the two sessions is established in S303and S304, and the other one in S309 and S310, the second embodiment justapplies one of the ways; another one is that the ICCF releases any oneof the sessions after S310, and the VMSC releases the other sessionafter receiving the release message to enhance the reliability.Moreover, even if the ICCF releases the two sessions between the UE-Band itself, the VMSC can also releases the other session after receivingthe message of releasing the first session, thus further enhance thereliability.

The above forwarding reason can be the called number field in the CAMELIDP message. For example, a different particular late call forwardingnumber in the CS domain is used as the identifiers of the forwardingreasons, and the particular late call forwarding number is filled intothe called number field in the CAMEL IDP message during the late callforwarding. Moreover, the forwarding reason may also be the forwardingreason field in the CAMEL IDP message.

Of course that this present invention may have several otherembodiments, and without departing from the spirit and essence of thepresent invention, those skilled in the field can make all kinds ofmodification or variations in accordance with the present inventionwhich should belong to the scope of the claims of the present invention.

Industrial Applicability

The present invention implements the late call forwarding with arelatively simple message exchanging process based on the existingnetwork architecture and the communication standard and sends theforwarding reason to the IMS domain by using the calling intelligentservice to control the late call forwarding services in the IMS domain,thus achieve the IMS centralized service control of the late callforwarding.

1. A late call forwarding method comprising, in an internet protocolmultimedia subsystem centralized service (ICS): A1, during a callsetting up, an internet protocol multimedia subsystem circuit switchcontrol function (ICCF) receiving a call request from a call sessioncontrol function (CSCF) serving a called user which registers in aCircuit Switching (CS) domain, the call being referred to as session A;the ICCF acquiring a called user's roaming number and initiating a call,being referred to as session B, to the called user; A2, after receivinga call request of the session B, a visited mobile switch center (VMSC)serving the called user determining that a late call forwarding isneeded according to a status of the called user and the called usersubscribes a late call forwarding service in the CS domain and the VSMChas calling customized application for mobile network enhanced logic(CAMEL) service data for the called user, then initiating a call to aforwarding target to trigger a calling CAMEL service; and A3: said VMSCexchanging CAMEL messages with said ICCF to inform said ICCF aboutsession associated information and a forwarding reason of the late callforwarding, and when said ICCF determines that the forwarding is needed,said ICCF sending a forwarding indication to the CSCF in response to thesession A or not responding to the session A depending on the forwardingreason, and said CSCF forwarding the forwarding indication to a telecomapplication server (TAS), and the TAS initiating a regular forwardingservice according to the received forwarding indication or triggeringthe call forwarding when receiving no response at an expiration, whereinthe VMSC is in the CS domain, and the ICCF, CSCF and TAS are in aninternet protocol multimedia subsystem.
 2. A late call forwarding methodof claim 1, wherein in the step A2, conditions for triggering said latecall forwarding comprises forwarding on user-determined user busy,forwarding on no paging response or on no answer.
 3. A late callforwarding method of claim 1, wherein in the step A3, said VMSCexchanging the CAMEL messages with said ICCF comprises the followingsteps of: A311. said VMSC sending the CAMEL message including thesession associated information and the forwarding reason to a globalsystem for mobile communications service control function (gsmSCF), andsaid gsmSCF forwarding the messages to said ICCF; and A312. after saidICCF determining that the message relates with the sessions A and Baccording to the session associated information in the received CAMELmessage, said ICCF sending a CAMEL failure message to said VMSC, thefailure message reaching said VMSC via said gsmSCF.
 4. A late callforwarding method of claim 1, wherein in the step A3, said VMSCexchanging the CAMEL messages with said ICCF comprises the followingsteps of: A321. said VMSC sending a CAMEL initial monitoring pointmessage including the session associated information and the forwardingreason to a global system for mobile communications service controlfunction (gsmSCF), and said gsmSCF forwarding the CAMEL initialmonitoring point message to said ICCF; A322. after said ICCF determiningthat the CAMEL initial monitoring point message associates with thesessions A and B according to the session associated information in thereceived CAMEL message, said ICCF allocating a temporary internetprotocol multimedia subsystem routing number (IMRN) and associating theIMRN with said sessions A and B, storing the forwarding reason, and thenreturning the IMRN to said VMSC through a CAMEL connection message, andsaid CAMEL connection message reaching said VMSC via said gsmSCF; A323.after receiving the CAMAL connection message, said VMSC taking said IMRNas a called party number to initiate a call being referred to session C;A324. after receiving the session C, said ICCF determining that thesession C associates with the sessions A and B according to said IMRNand that the forwarding is needed, then sending said correspondingindication in the response message to said CSCF within the session A. 5.A late call forwarding method of claim 4, wherein in the step A323, whensaid VMSC takes said IMRN as the called party number to initiate thesession C, said VMSC first sends an integrated services digital networkuser part (ISUP) initial address message which reaches a media gatewaycontrol function (MGCF) first; said MGCF sends a session initiationprotocol (SIP) session request message to said ICCF, and the calledparty number is said IMRN, and said message reaches said ICCF via saidCSCF, thereby said ICCF receives said session C.
 6. A late callforwarding method of claim 3, wherein in the step A311, said CAMELmessage including the session associated information and the forwardingreason is an Initial Detection Point (IDP) message.
 7. A late callforwarding method of claim 6, wherein said forwarding reason isindicated in a field of the called party number or a forward reason insaid IDP message.
 8. A late call forwarding method of claim 6, whereinsaid session associated message is a calling party number or an originalcalled party number in the IDP message; alternatively, said sessionassociated message is the calling party number and the original calledparty number in the IDP message.
 9. A late call forwarding method ofclaim 3, wherein the step A3 applies the following way to release thesession B: After determining that the received CAMEL message associateswith the sessions A and B, said ICCF releasing the session B;Alternatively, after receiving a customized applications for mobilenetwork enhanced logic (CAMEL) failure message from said ICCF, said VMSCreleasing the session B; Alternatively, after said ICCF determines thatthe received CAMEL message associates with the sessions A and B, saidICCF releasing the session B; meanwhile, after said VMSC receives theCAMEL failure message from said ICCF, said VMSC also releasing thesession B.
 10. A late call forwarding method of claim 4, wherein thestep A3 applies the following way to release the sessions B and C: Afterdetermining that the session C associates with the sessions A and B,said ICCF releasing the sessions B and C; Alternatively, after said ICCFdetermines that the session C associates with the sessions A and B, saidICCF releasing one of the sessions B and C, and after said VMSC receivesthe release message, said VMSC releasing the other session;Alternatively, after said ICCF determines that the session C associateswith the sessions A and B, said ICCF releasing the sessions C and B; andafter said VMSC receives a first arriving release message of one of thesessions C and B, said VMSC releasing the other sessions.
 11. A latecall forwarding method of claim 2, wherein in the step A3, when theforwarding reason is call forwarding on no answer, said ICCF returnscorresponding session initiation protocol (SIP) failure response messageto the CSCF, and after said TAS receives the message, said TAS initiatesa call forwarding on no answer service according to said SIP failureresponse message; alternatively, said ICCF does not send any responsemessage to the TAS, and the TAS waits for an expiration and triggers thecall forwarding on no answer; Alternatively, if the forwarding reason iscall forwarding on user-determined user busy or call forwarding on nopaging response, said ICCF returns a corresponding SIP failure responsemessage to the CSCF, and after said TAS receives the message, said TASinitiates the call forwarding on user-determined user busy or the callforwarding on no paging response according to the SIP failure responsemessage.
 12. A late call forwarding method of claim 1, wherein in thestep A2, when the called user registers in a circuit switching domain,subscription data of the called user including late forwarding servicedata and the Originating-CAMEL Subscription Information(O-CSI) aredownloaded to said VMSC, and said VMSC triggers said late callforwarding service and the CAMEL service according to the subscriptiondata and the status of the called user.
 13. A late call forwardingmethod of claim 4, wherein in the step A321, said CAMEL messageincluding the session associated information and the forwarding reasonis an Initial Detection Point (IDP) message.